Multi-channel radiometer imaging system

ABSTRACT

A radiometer includes a housing and an RF board carried by the housing. A hybrid and amplifier circuit receives an unknown signal and a known reference signal. A switch receives signals as inputs from the hybrid and amplifier circuit and switches between the inputs. A detection circuit receives and detects the signal from the switch forming a detected signal. A controller board is carried by the housing and an integration circuit and processor receive the detected signal and produces a digital output.

RELATED APPLICATIONS

This application is a continuation of Ser. No. 10/995,952 filed Nov. 23,2004, which is a continuation-in-part of Ser. No. 10/847,892 filed onMay 18, 2004, which is based on provisional application Ser. No.60/504.182 filed Sep. 18, 2003, the disclosures of which are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to the field of focal plane radiometers,and more particularly, the present invention relates to radiometersystems applicable for use at millimeter wave (MMW) frequencies.

BACKGROUND OF THE INVENTION

Since radio waves may be considered infrared radiation of long wave, ahot body would be expected to radiate microwave energy thermally. Inorder to be a good radiator of microwave energy, a body must be a goodabsorber. The best thermal radiator is a “black body.” The amount ofradiation emitted in the MMW range is 10⁸ times smaller than the amountemitted in the infrared range. Current MMW receivers, however, have atleast 10⁵ times better noise performance than infrared detectors, andwith some temperature contrast, the remaining 10³ may be recovered. Thismakes passive MMW imaging comparable in performance with currentinfrared systems. This unique characteristic makes MMW radiometers apopular choice for sensing thermal radiation. MMW radiometers have beenused in many different applications such as remote terrestrial andextra-terrestrial sensing, medical diagnostics and defense applications.MMW electromagnetic radiation windows occur at 35 GHz, 94 GHz, 140 GHzand 220 GHz. The choice of frequency depends on specific applications.

Focal plane arrays are used to form images from radiation received by areflector antenna. Millimeter wave (MMW) focal plane array radiometersalso have been used in many applications to form images based on thermalsensing of radiated microwave energy. The sensitivity of existingradiometer designs, however, has been limited to about 1 deg K,resulting in poor images.

The principle of operation of the radiometric technique is fullydescribed in the literature. The design of a typical radiometer is basedon the technique of comparing the level of electromagnetic noise emittedby an unknown source to a reference or stable noise source. Thistechnique and devices were initially proposed by Dicke [R. H. Dicke,“The Measurement of Thermal Radiation at Microwave Frequencies,” TheReview of Scientific Instruments, Vol. 17, No. 7, Jul. 1946].

In a Dicke radiometer circuit, the signals from an antenna are sampledand compared with signals from a reference source maintained at a knownconstant temperature. This overcomes some of the problems of amplifierinstability, but in general does not alter effects resulting fromimperfect components and thermal gradients.

While other types of radiometric devices have been used with somesuccess, the Dicke (or comparison) type of radiometer has been the mostwidely used for the study of relatively low level noise-like MMWsignals, especially where the noise signals to be examined are oftensmall in comparison to the internally generated noise level within theradiometer receiver. While there are several types of comparisonradiometers, one popular type of radiometer for use in themicrowave/millimeter wave frequency bands is that in which an incomingsignal to be measured and a standard or calibrated reference noisesignal are compared. This type of radiometer consists essentially of thecomparison of the amplitude of an unknown noise signal coming from thesource to be examined with a known amplitude of a noise signal from acalibration source. This method has been found useful in measuring withconsiderable accuracy the effective temperature of an unknown source.

In the Dicke or comparison type radiometer, the receiver input isswitched between the antenna and a local reference signal noisegenerator. The detected and amplified receiver output is coupled to aphase-sensing detector operated in synchronism with the input switching.The output signal from such a radiometer receiver is proportionate tothe difference between the temperature of the reference signal sourceand the temperature of the source viewed by the antenna inasmuch as thephase-sensing detector acts to subtract the background or internal noiseof the receiver.

A Dicke radiometer uses an RF switch coupled between an antenna and aradiometer receiver, allowing the receiver to alternate between theantenna and a known reference load termination. The receiver output isconnected to a synchronous detector that produces an output voltageproportional to a difference between the antenna and the referencetemperature. Null balance operation for the Dicke radiometer has beenachieved by coupling in noise from a hot noise diode to the antenna portof the RF switch thereby enabling matching the temperature from standardreference loads.

The sensitivity of radiometer measurements are also often limited byrandom gain fluctuations in the RF front end, low frequency noise (1/f),and bias in the detector circuits. Over the last decades many specialtechniques, including Dicke switching, have been implemented to reducemeasurement errors. Many of these proposals do not yield a true solutionthat will allow MMW radiometers to be commercially viable. In addition,the high cost of MMW RF receivers has limited the number of channels inthe radiometer to a low number, resulting in a requirement to scan bothazimuth and elevation to create an image.

The invention disclosed in the commonly assigned and incorporated byreference patent application Ser. No. 10/847,892 eliminates the need fora Dicke switch and does not use a synchronizing circuit because it usesthe source and reference all the time, and runs the source and referencesignal through the amplifiers. It used a balanced channel approach andMMIC chips. Thus, a radiometer channel can be implemented by the use ofeither a single millimeter wave monolithic integrated circuit (MMIC) orthrough discrete implementation using printed hybrids and multiple MMIClow noise amplifiers (LNA's).

This compact radiometer as disclosed can fit directly into the antennafocal plane. A quadrature hybrid network is used in the front end todistribute RF input signals and reference signals to a balancedamplifier chain, thereby reducing gain variations and improvingradiometer sensitivity. A balanced detector diode circuit, for example,a pair of diodes in one non-limiting example, eliminates drift errorsintroduced by a detector diode as a function of temperature.

A video signal chopper amplifier circuit, also referred to by some as aauto zero amplifier, eliminates bias introduced by the video amplifier.A near perfect channel-to-channel matching exists through the use ofquadrature hybrid network or through digital signal processingcorrections. This hybrid radiometer provides improved sensitivity overthe Dicke radiometer.

This radiometer system, however, requires processing of two channels,i.e., the antenna and reference, resulting in higher system complexityand cost. It would be advantageous to provide a radiometer design thatcould combine features of the different radiometers to achieve lowsystem temperature and low implementation costs.

SUMMARY OF THE INVENTION

The radiometer of the present invention uses a combination of hybrid,low noise amplifiers (LNA's) and a switch to achieve low systemtemperature and low implementation cost. Unlike a typical Dickeradiometer where the switch losses translate directly into an increasein system noise, such as system temperature, the switch is positionedafter a low noise amplifier. This design eliminates the impact of theswitch losses. The radiometer of the present invention also combines thebenefit of the low noise figure achieved by the hybrid approach and thesimple, single channel processing achieved by the Dicke approach.

In accordance with the present invention, a radiometer system includes ahybrid and amplifier circuit that receives an unknown signal and knownreference signal. A switch receives the unknown signal and knownreference signal as inputs from the hybrid and amplifier circuit andswitches between the signals. A detection circuit receives and detects asignal from the switch forming a detected signal. A controller circuitreceives the detected signal and produces a digital output.

In one aspect of the present invention, the hybrid and amplifiercircuit, the switch and the detection circuit comprise a radiometerfront end. This radiometer can be sized and of such weight to fitdirectly into an antenna focal plane. The hybrid and amplifier circuitpreferably is formed as printed hybrids and low noise amplifiers. Thecontroller circuit is formed as an integrator circuit. The controllercircuit can be formed as an analog-to-digital converter for convertingthe detected signals to digital signals. This controller circuit canalso be formed as a processor that receives the digital signals from theanalog-to-digital converter for further processing.

In yet another aspect of the present invention, the hybrid and amplifiercircuit and switch are formed as a MMIC chip. The amplifier circuitreceives a signal from the switch before the detection circuit. Theamplifier circuit is also formed as a MMIC chip and is formed as twoamplifier circuits and a filter circuit between the switch and detectioncircuit. Each amplifier circuit is also formed as a MMIC chip.

In yet another aspect of the present invention, the radiometer systemincludes an RF board having the hybrid and amplifier circuit, switch anddetection circuit thereon. A controller board has the controller circuitand is operative with components on the RF board. A housing can carrythe RF board and controller board. In one aspect of the presentinvention, the RF board is preferably formed from a soft board orceramic material.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the detailed description of the invention whichfollows, when considered in light of the accompanying drawings in which:

FIG. 1 is a fragmentary environmental view of a typical radiometerantenna system for a focal plane array.

FIG. 2 is a block diagram showing a receiver front end in a typicalradiometer system.

FIG. 2A is a block diagram showing how radimeter modules are typicallyconnected to the antenna with a waveguide manifold in current art.

FIG. 3 is a block diagram illustrating the basic functional componentsof the radiometer of the present invention.

FIG. 3A is a block diagram of the quadrature hybrid used in theradiometer of FIG. 3 showing how inputs A, B are divided equally in thefirst hybrid, then reconstructed in the second hybrid.

FIG. 3B is a block diagram showing a two-stage MMIC LNA chip of thepresent invention as a representative example.

FIG. 3C is a block diagram showing a three-stage MMIC LNA chip of thepresent invention.

FIG. 3D is a block diagram illustrating the basic functional componentsof the radiometer of the present invention using the MMIC chip of FIG.3B.

FIG. 4 is a block diagram showing functional components of anotherexample of a multi-channel radiometer of the present invention.

FIG. 5 is a block diagram showing the layout for the RF front end in theradiometer of the present invention.

FIG. 6 is a plan view showing a multi-channel radiometer layout on asingle RF board.

FIG. 7 is an exploded isometric view of a compact multi-channelradiometer module showing a base housing, RF board, controller board andtop cover.

FIG. 8 is an isometric view of the assembled multi-channel radiometermodule of the present invention.

FIG. 9 is a top plan view of the multi-channel millimeter waveradiometer module shown in FIG. 8.

FIG. 10 is a chart showing Dicke radiometer sensitivity of the typeshown in FIG. 2.

FIG. 11 is a chart showing the radiometer sensitivity of the presentinvention.

FIG. 12 is a block diagram illustrating basic functional components ofthe radiometer system of the present invention and showing a hybrid lownoise amplifier followed by the switch.

FIG. 13 is a chart showing a Dicke radiometer system and noiseperformance of the different components.

FIG. 14 is a chart showing radiometer system noise performance of ahybrid radiometer system.

FIG. 15 is a chart showing the radiometer system noise performance for ahybrid/switch radiometer of the present invention.

FIG. 16 is a block diagram of a hybrid/switch low noise amplifierradiometer of the present invention showing its use of MMIC chips.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout, and prime notation is used toindicate similar elements in alternative embodiments.

The present invention overcomes many existing shortcomings of currentradiometers, including gain variation of the amplifiers, low frequencynoise, detector bias, low sensitivity and high cost. The presentinvention reduces cost and size of a radiometer by a least a factor often and provides a commercial advantage over many current radiometers.

The low cost MMW radiometer of the present invention includes a housingsection, a multi-channel RF board, and a controller board. The housingsection is preferably made up of a base metal housing and a metallizedplastic or metal cover that includes the RF launch opening, typicallyfilled with a dielectric material, for example, a plastic material toallow for less than one wavelength spacing between the sensingradiators. The RF board preferably is formed from a single soft board orceramic material. All MMW microstrip circuits, for example, 50 Ohmlines, filters, 90° hybrids and RF radiators, are printed on this board.MMIC amplifiers can be either attached directly to the board or, throughcut-outs, on a carrier plate underneath to the RF board. A simple MMICchip can also be used. A controller board, which uses a low costmicrocontroller, performs any necessary video signal amplification,digitization and conditioning, automatic RF amplifier bias adjustment,and DC power regulation. The controller board interfaces with anexternal display system.

The compact radiometer of the present invention can fit directly in theantenna focal plane. A quadrature hybrid network is used in the frontend to distribute RF input signals and reference signals to a balancedamplifier chain, thereby reducing gain variations and improvingradiometer sensitivity. A balanced detector diode circuit, for example,a pair of diodes in one non-limiting example, eliminates drift errorsintroduced by a detector diode as a function of temperature.

A video signal chopper amplifier circuit, also referred to by some as aauto zero amplifier, eliminates bias introduced by the video amplifier.A near perfect channel-to-channel matching exists through the use ofquadrature hybrid network or through digital signal processingcorrections.

FIG. 1 shows a typical radiometer antenna system 20. The main antenna 22collects temperature data or other pertinent data to be analyzed. Thedata is focused in the middle of the antenna at the focal plane array 24using a sub-reflector 26.

FIG. 2 shows a common prior art “Dicke” type radiometer system 30,including a receiver front end. In a Dicke radiometer, generally areceiving circuit detects weak signals in noise and modulates thesesignals at an input. The circuit demodulates the signals and comparesthe output with a reference from the modulator. Coincidence indicates asignal presence. For example, microwave noise power can be measured bycomparing it with the noise from a standard source in a waveguide.

In this illustrated example of a Dicke radiometer, the antenna 32 sensestarget temperature, which is proportional to the radiated target energy.The energy passes through a Dicke switch 34 of the type known to thoseskilled in the art and into a series of MMIC amplifiers 36 a, 36 b, 36c. A band pass filter 38 sets the receiver bandwidth. A square lawdetector 40 detects the signal and passes it to an integrator 42, whichsums the signal over an observation period. A data acquisition andprocessing circuit 44 receives the integrated signal, where it isdigitized, compensated for gain variation, and processed for display ona video or for further processing. To cancel the effects of gainvariation, the Dicke switch 34 samples a reference source 46. Gainvariations in the receiver are cancelled using the measured referencegain.

Radiometer sensitivity is important. The precision in estimating themeasured temperature is often referred to as the radiometer sensitivity,ΔT. This parameter is a key quantity characterizing the performance of aMMW radiometer. In radiometer terminology, this is the smallest changein temperature that can be detected by the radiometer. The equation,which derives the sensitivity of the system 30 shown in FIG. 2 is:P _(sys) =P _(A) +P _(rec)

where P_(sys)=total input power

P_(A)=Noise power at the antenna=k T_(A) B

_(prec)=Noise power generated in the receiver=kT_(rec)B

K=Boltzmann's constant

B=receiver bandwidth

Assuming a square law detector, the radiometer output voltage is anaverage value of the radiometer output noise power. The square lawdetector can have an output proportional to the square of the appliedvoltage, e.g., the output is proportional to the square of the inputamplitude. A radiometer output voltage is:V _(out) =P _(sys) ×G _(sys)where G_(sys) is the receiver gain.

Assuming that G_(sys) and T_(rec) are constant, the radiometersensitivity is:ΔT _(ideal)=(1√{square root over (Bτ)})T _(sys)where τ is the integration time.

In most applications, however, G_(sys) and T_(rec) are not constant, andtheir variations cause degradation of the radiometer sensitivity asfollows:

Gain variations effects:ΔT _(G)=(T _(A) −T _(ref))×(ΔG _(sys) /G _(sys))

Assuming a five degree difference between the antenna temperature andthe reference temperature, a +/−3 dB gain variation (over the 3 LNA's 36a, 36 b, 36 c), and a 40 dB total system gain, the radiometersensitivity will vary by about 5%.

Temperature variation effects can be shown:ΔT _(ant)=(T _(A) +T _(rec))/(√{square root over (Bτ/2)})=√{square rootover (2)}(T _(A) +T _(rec))/(√{square root over (Bτ)})ΔT _(ref)=(T _(ref) +T _(rec))/(√{square root over (Bτ/2)})=√{squareroot over (2)}(T _(ref) +T _(rec))/(√{square root over (Bτ)})Assuming statistical independence, the temperature variation can beshown:

$\begin{matrix}{{\Delta\; T} = \lbrack {( {\Delta\; T_{G}} )^{2} + ( {\Delta\; T_{ant}} )^{2} + ( {\Delta\; T_{ref}} )^{2}} \rbrack^{\frac{1}{2}}} \\ {= {\frac{\lbrack {{2( {T_{A} + T_{rec}} )^{2}} + {2( {T_{ref} + T_{rec}} )^{2}}} }{( {B\;\tau} )^{\frac{1}{2}}} + {( {\Delta\;{G_{sys}/G_{sys}}} )^{2}( {T_{A} - T_{ref}} )^{2}}}} \rbrack^{\frac{1}{2}}\end{matrix}$Assuming a balanced Dicke radiometer (i.e. T_(A)=T_(ref)), the aboveequation can be simplified to:

$\begin{matrix}{{\Delta\; T} = {2{( {T_{A} + T_{rec}} )/\sqrt{B\;\tau}}}} \\{= {2\Delta\; T_{ideal}}}\end{matrix}$

Therefore, the Dicke radiometer sensitivity is twice that of an idealtotal power radiometer. The factor of two (2) comes about because theDicke switch alternates between the reference and the antenna such thatTA is observed for only half of the time.

FIG. 2A shows how the radiometer channels 48, indicated as channels 1 .. . N, as part of RF modules, are typically connected to the antenna 48a. Because of the large size of the radiometer RF modules, which cannotfit directly in the antenna focal plane, a waveguide manifold 48 b isused to connect the modules to the focal plane. The waveguide manifold48 b increases the front end losses by at least 2 dB, resulting inreduced radiometer sensitivity. The channels 48 connect to dataacquisition and processing circuit 48 c.

FIG. 3 is a block diagram of the radiometer 50 of the present invention.This radiometer design does not use a Dicke switch, yet it stilldelivers superior sensitivity and can be readily manufactured.

A radiator 52 provides a first signal input A while a reference 54provides a second signal input B. The radiator 52 could be many types ofradiator elements used in radiometers, including an antenna. Microstripquadrature hybrid circuits 56 are operable with low noise amplifiercircuits 58. The hybrid circuits can be 90° hybrids. Bandpass filtercircuits 60 a, 60 b receive the signals represented at A and B, whichare output to detector circuits 62 a, 62 b. These components aretypically mounted on an RF board indicated by the dashed lines at 64.The RF board is typically formed from a single soft board or ceramicmaterial. All MMW microstrip circuits, for example, 50 ohm lines,filters, hybrids and RF radiators, are printed on this board. Any MMICamplifiers can be attached directly to the board, or through cut-outs,on a carrier plate underneath to the RF board.

The signals (A and B) are output to a controller board indicated bydashed lines at 70. On this board, any necessary video signalamplification, digitization and conditioning, automatic RF amplifierbias adjustment, and DC power regulation occurs. This board caninterface directly with a video display system. The signal is receivedat two chopper amplifier circuits 72 a, 72 b. After amplification, thesignals are integrated at integrator circuits 74 a, 74 b, and digitizedat analog/digital (A/D) circuits 76 a, 76 b. A microcontroller circuit78 provides digital video processing and receives an antenna temperaturesignal 80, amplifier control signal 82, and reference temperature signal84. The output from the microcontroller circuit 78 is sent to a displayor other external sensors 86.

The radiometer 50 of the present invention uses microstrip quadraturehybrids 56 to distribute the signal and reference powers to the balancedamplifier chain as illustrated. The pairs of low noise amplifiers(LNA's) 58 are cross-coupled to each other, similar to a conventionalbalanced amplifier configuration.

The quadrature hybrid shown in FIG. 3A is a well known four-port devicethat splits the energy into equal parts at the output, but with a 90degree phase difference. For example, the signal A at the input port P1of the hybrid is divided up to two parts at the output ports P3 and P4.The same is true for the input signal B at the input port P2 of thehybrid, which is also divided equally at the output ports P3 and P4.When the output of the first hybrid is used as input into a secondhybrid, the signals A and B are restored at the output of the secondhybrid (of course with some losses due to the hybrids). The two inputs Aand B, which can represent the antenna port and the reference port, orrepresent two antenna ports representing two different polarizations,are divided equally among the amplifiers and reconstructed at theoutput, as shown in FIG. 3A. One other unique feature of this hybriddesign is that failure of one or more of the LNA's 58 in the chain doesnot result in failure of the channel itself. Because of the distributedgain approach, the gain of the channel will drop by a small amount,which can be accounted for in the microcontroller 78. This is differentfrom the traditional radiometer shown in FIG. 2, where failure of oneLNA will result in total failure of the element.

Because each signal passes through each amplifier in the chain, anyfluctuation in the gain of any of the amplifiers is applied equally toboth signals (T_(A) & T _(Ref)) Assuming that the hybrid circuits 56 arewell balanced by using good design practices, this radiometer designguarantees that the gain in each channel is substantially the same. Inaddition, because the gain in each channel is essentially the average ofthat of all the amplifiers in the chain, the overall gain fluctuation iseffectively reduced by a factor of the square root of N, where N is thenumber of amplifiers.

${\Delta\; G_{sys}} = \lbrack {( {1/N} ){\sum\limits_{I = 1}^{N}\;( {\Delta\; G_{i}} )^{2}}} \rbrack^{\frac{1}{2}}$

Assuming the same amount of the LNA's gain variation (+/−3 dB) used forthe Dicke radiometer as shown in FIG. 2, the radiometer system gainvariation of the present invention will be only about +/−0.7 dB.Therefore, it is evident that the radiometer of the present inventionprovides the inherent benefits of receiver gain fluctuation reductionand guarantees equal gain for both the antenna power and the referencesignal. This feature provides the same benefit as the Dicke switchwithout the added losses and the complex switching circuitry. Also, theabsence the Dicke switch in the present invention allows continuousobservation of the antenna temperature, thereby achieving thesensitivity of a total power radiometer.ΔT _(ideal)=(1/√{square root over (Bτ)})T _(sys)

Using commercially available W-band LNA's with over 20 GHz bandwidth,such as an ALH394 circuit made by Velocium of Redondo Beach, Calif., andassuming an integration time of 20 msec and 1200 K total systemtemperature, this radiometer sensitivity is less than 0.1 degree. TheALH394 is a broadband, three-stage, low noise monolithic HEMT amplifier.It has a small die size and is passivated. Bond pad and backsidemetallization can be Ti/Au and compatible with conventional die attach,thermocompression and thermosonic wire bonding assembly. It can have ausable radio frequency of 76 to about 96 GHz, linear gain of about 17dB, and a noise figure of about 5 dB depending on applications. It canuse DC power of about 2 volts at 34 mA. Bond pads can include VG1, VG2and VG3, VD1, VD2, VD3, with an RF in and RF out pad.

The RF signals at the output of the band pass filter 60 a, 60 b aredetected using the square law detector 62 a, 62 b. In order to eliminateany detector variation over temperature, a pair of balanced diodes 62 a,62 b, such as a DBES105a diode manufactured by United MonolithicSemiconductors, can be used. This dual Schottky diode is based on a lowcost 1 μm stepper process with bump technology and reduced parasiticconductances and having a high operating frequency. It can be aflip-chip dual diode with high cut-off frequencies of about 3 THz and abreakdown voltage of less than −5 volts at 20 uA. It has a substantiallyadequate ideality factor of about 1.2.

The diodes output an equal amount of power, but with opposite polarity.This method effectively cancels any bias or drift caused by the diodes.The very small DC voltages at the output of the diodes are typicallyvery difficult to amplify accurately. DC offsets introduced by theop-amps are usually a cause of the problem, aggravated often by lowfrequency noise (1/f). The radiometer 50 of the present invention useschopping op-amp circuits 72 a, 72 b, also known as auto zero amplifiers,such as the AD8628 amplifier manufactured by Analog Devices. Thisamplifier circuit eliminates DC offset and low frequency (1/f) noise.

The AD8628 amplifier has ultra-low offset, drift and bias current. It isa wide bandwidth auto-zero amplifier featuring rail-to-rail input andoutput swings and low noise. Operation is specified from 2.7 to 5 voltssingle supply (1.35V to 2.5V dual supply). It has low cost with highaccuracy and low noise and external capacitors are not required. Itreduces the digital switching noise found in most chopper stabilizedamplifiers, and has an offset voltage of 1 μV, a drift less than 0.005μV/° C., and noise of 0.5 uV P—P (0 Hz to 10 Hz). This amplifier isavailable in a tiny SOT23 and 8-pin narrow SOIC plastic packages.

An offset voltage of less than 1 μV allows this amplifier to beconfigured for high gains without risk of excessive output voltageerrors. The small temperature drift of 2 nV/° C. ensures a minimum ofoffset voltage error over its entire temperature range of −40° C. to+125° C. It has high precision through auto-zeroing and chopping. Thisamplifier uses both auto-zeroing and chopping in a ping-pong arrangementto obtain lower low frequency noise and lower energy at the chopping andauto-zeroing frequencies. This maximizes the signal-to-noise radio (SNR)without additional filtering. The clock frequency of 15 kHz simplifiesfilter requirements for a wide, useful, noise-free bandwidth. Theamplifier is preferably packaged in a 5-lead TSOT-23 package.

1/f noise, also known as pink noise, is a major contributor of errors indecoupled measurements. This 1/f noise error term can be in the range ofseveral μV or more, and when amplified with the closed-loop gain of thecircuit, can show up as a large output offset. 1/f noise is eliminatedinternally. 1/f noise appears as a slowly varying offset to inputs.Auto-zeroing corrects any DC or low frequency offset, thus the 1/f noisecomponent is essentially removed leaving the amplifier free of 1/fnoise.

The output of the integrator circuits 74 a, 74 b for both the antennasignal and the reference signals are digitized using highly linear A/Dcircuits 76 a, 76 b and are sent to the microcontroller 78, where thereference signal is subtracted from the antenna signal to obtain theactual target temperature. The microcontroller 78 can monitor thetemperature of the antenna through a sensor attached to the antenna. Anydifferences between the antenna and the reference are accounted for andcorrections are applied appropriately in software. The microcontroller78 also controls the LNA bias and monitors the amount of current drawnby each amplifier and adjusts the amplifier gain.

FIG. 3B shows a two-stage MMIC chip 112 that can be used in the presentinvention to replace the discrete implementation of the hybrid andcascade LNA's shown in FIG. 3. This MMIC LNA chip receives a signal fromthe antennp 114 or reference load 116 that enters through signal inputsA and B into the hybrid circuit 118 and into amplifiers 120 a. 120 bthrough amplifiers 124 a, 124 b, through hybrid 122 to be output assignals A and B amplified.

FIG. 3C is a block diagram showing a three-stage MMIC LNA chipimplementation 125 with respective amplifier circuits 128 a and 128 b.

Thus, the balanced channel approach of the present invention can useMMIC chips and the implementation of a radiometer channel can occureither by the use of a single millimeter wave monolithic integratedcircuit (MMIC) or through discrete implementation using printed hybridsand multiple MMIC LNA'S.

FIG. 3D is a block diagram of the radiometer 50 of the presentinvention. This radiometer design uses the MMIC chip 112 shown in FIG.3B to replace the printed hybrids and individual LNA chips. This figureshows yet another example of how this invention can be used to buildradiometer channels. Only one detector 62 as a pair of diodes is used.One chopper 72 amplifier and any integrator 74 and A/D circuit 76.

The radiometer of the present invention can also be manufactured in anarrangement having a larger number of channels, such as shown in FIG. 4.Prime notation is used to show the various radiators 52′, hybridcircuits 56′, and low noise amplifiers 58′. As illustrated, signals A, Band C are generated from radiators 52′ and a reference signal isgenerated from the reference 54′. Two parallel hybrid circuits 56′ areillustrated at the front end and input into four parallel, low noiseamplifier circuits 58′ instead of two as shown in FIG. 3. This followsby other parallel hybrid circuits 56′ and low noise amplifier circuits58′. Four bandpass filters 60′ are illustrated with detector circuit 62′forming a three-element radio frequency module.

FIG. 5 shows an example of a layout for the RF front end used in theradiometer 50 of the present invention, forming a radiometer cell 90.The radiator elements 52, the quadrature hybrids 56, 50 ohm microstriplines 59 and the filters 60 a, 60 b are all printed on a soft board or aceramic board. Isolation vias 100 are used to isolate the amplifiers 58and reduces the likelihood of oscillations.

FIG. 6 shows a multi-channel radiometer layout on a single RF board. Aplurality of radiometer cells 90 are illustrated, forming a N elementarray 102 with channels 110. Radiators 52 are also illustrated. Thisdesign approach allows for low cost implementation of a large number ofchannels. The radiator elements 52 can be spaced half a wavelength (λ/2)apart for lower cross coupling, lower sidelobes and overall improvedoperations. The channels 110 are stacked on both sides of the board inorder to achieve two rows 1110 a, 10 b of radiometer cells 90 in a verysmall amount of space. For dual polarization applications, one row 110 amay be vertically polarized while the second row 10 b could behorizontally polarized. The radiators, for example as antenna elements,can be alternated between vertical and horizontal polarization in thesame row. For example, a 32×2 array can easily fit a 3×4 inch RF board.This board can become part of a radiometer module of the presentinvention.

FIG. 7 shows an exploded view of a compact, multi-channel radiometermodule 130 of the present invention. A base housing 131, typicallymade-up of aluminum, is used to receive the RF board 64, which can beattached to a CTE matched carrier 132. The controller board 70, whichsupplies all the DC voltages and control signals, makes contact with theRF board 64 through the use of DC contact connectors. The top cover 134,which can be made from a plastic material, is metallized everywhereexcept where the radiator areas 136 correspond to the location of theradiators. These unmetallized radiator areas 136 provide a dielectricmedia for the RF energy to travel through. Thus, RF launch openings areformed. A slot 70 a in the controller board provides access to theantenna elements. The entire unit is assembled using fasteners, such asscrews received in fastener apertures 138 a.

FIG. 8 shows fully assembled multi-channel radiometer module of thepresent invention forming a radiometer module or “sensor package” asillustrated.

FIG. 9 is a top plan view of the multi-channel millimeter waveradiometer module shown in FIG. 8 and showing the radiator areas 136 andradiators. FIGS. 10 and 11 show various components of a Dicke radiometer(FIG. 10) and showing the Dicke radiometer sensitivity as compared tothe radiometer sensitivity of the present invention as shown in FIG. 11.The different components of the radiometers and the relative sensitivityand operating or reference values are shown under the specific elementsas illustrated.

The radiometer module of the present invention has at least six timeshigher sensitivity than more current radiometer sensitivity, such as theDicke radiometer sensitivity explained above with reference to FIG. 2and shown in FIG. 10. The present invention is advantageous and providesa radiometer with the sensitivity of less than 1° K. The radiometermodule (sensor) is at least ten times smaller than other radiometerscurrently in use. The radiometer of the present invention also is atleast ten times lighter in weight than any other radiometer inexistence, which typically weighs no less than 20 pounds. The radiometerof the present invention is typically less than about three pounds.

The present invention also is self-correcting for temperature and gainvariations. It uses the balanced pair of diodes for detection and thechopper operational amplifiers to eliminate any bias and reduce 1/Fnoise. The microcontroller can monitor temperature changes between theantenna and the reference by reading any temperature sensors located onthe antenna and near the reference. This can be based on temperatures toadjust the correction factor. The gain can be continuously monitored andthe bias adjusted of the low noise amplifier (LNA) to maintain theconstant gain. Real-time corrections can be performed on all videochannels to account for any changes in temperature or gain.

The radiometer of the present invention also has self-healing capabilitybecause of the distributed gain approach. Failure of one or more LNA'sin each channel will not result in failure of the channel. Themicrocontroller can compensate for the drop of any amplifiers in thechain.

FIG. 12 is a block diagram of a radiometer system 200 of the presentinvention using a combination of hybrid low noise amplifiers and aswitch to achieve low system temperature and low implementation cost.Unlike a typical Dicke radiometer where the switch losses translatedirectly into an increase in system noise figures, and systemtemperature. Moving the switch after a first low noise amplifier circuitas illustrated in FIG. 12 nearly eliminates the impact of the switchlosses. The radiometer system of the present invention combines thebenefit of the low noise figure achieved by the hybrid approach and asimple, single channel processing achieved by the Dicke approach.

FIG. 12 shows an antenna feed 202 operatively connected to the low noiseamplifier circuit 204, which receives antenna signal and the referencesignal from the reference load 206. The low noise amplifier circuit 204is cross-coupled and quadrature and has signal outputs received at theswitch 208 that also receives a signal from a switch control 210. Thiscircuit could be a microcontroller or other processor as part of orcontrolled from the controller board. The switch 208 is operativelyconnected to an adjustable low noise amplifier 212, which receives again control signal from gain control circuit 214. The output signalenters a band pass filter 216, low noise amplifier circuit 218 anddetector circuit 220, all of the type that could be used as describedfor the embodiment shown in FIG. 3 or other embodiments as non-limitingexamples. The components as described are contained on an RF board 222shown by the dashed line.

The detected signal from the detector circuit 220 passes into anintegrator circuit 224 that is positioned on the controller board 226and into a sample and hold circuit 228. The analog signal is thenconverted into a digital signal by an appropriate analog/digitalconverter 230 and received in a microcontroller 232 which is operativewith a C&M circuit 234. A power regulation circuit 236 receives DCsignals from a DC source 238 and regulates the RF board 222 andcontroller board 226. The microcontroller 232 can output a digitalsignal to a display or other external sensors 240 as described before.It should be understood that the term microcontroller in thisdescription encompasses many different types of controllers andprocessors.

FIGS. 13, 14 and 15 show respective comparison of the resulting systemnoise performance from a traditional Dicke radiometer and the hybrid andthe hybrid/switch of the present invention. The left hand side of thechart shows the gain/loss in decibels, followed by the noise figure indecibels, the noise temperature in degrees Kelvin, the cumulative gain,the cumulative NF, and the cumulative temperature. Each of thefunctional blocks of the radiometer is illustrated with correspondingresults listed under that corresponding functional block. For example,FIG. 13 shows the ambient temperature and the various results at thehorn, the waveguide transition (WGT), the switch, the two low noiseamplifiers, the band pass filter, the low noise amplifier and thedetector.

FIG. 14 shows the results for the hybrid circuit described before, suchas with FIG. 3, with the ambient temperature and the horn, the waveguidetransition, the low noise amplifier, microstrip, low noise amplifier,band pass filter, low noise amplifier microstrip and detector.

FIG. 15 shows the results for the hybrid and switch combination of thepresent invention, showing the ambient temperature, and the functionalblocks of the horn, waveguide transition, the low noise amplifier thatreceives the reference signal, the switch, the microstrip, the low noiseamplifier, the band pass filter, the low noise amplifier and detectorcircuit.

These results show the system temperature in degrees Kelvin (K) for thethree configurations corresponding to the Dicke, hybrid andhybrid/switch. The Dicke radiometer in the example as shown in FIG. 13results in 2129.9 degrees K system temperature. The hybrid radiometerresults in 1337.2 degrees K system temperature. The hybrid/switchradiometer of the present invention results in 1361.2 degrees K. Thus,the hybrid/switch system of the present invention results in only a 24degree K increase in system temperature over the hybrid approach, butbenefits from the single channel process and simplicity of the Dickeradiometer.

FIG. 16 is another block diagram of the radiometer of the presentinvention showing how different MMIC chips can be implemented in thecombined hybrid/switch radiometer front end of the present invention. Asillustrated, signals from the antenna 202 are received in the first MMICchip 250, which includes a hybrid 252, amplifiers 254, hybrid 256 andnon-reflective switch 208. A second MMIC 260 chip includes amplifiers262. This is followed by a filter 264 and a third MMIC chip 266 withappropriate amplifiers 268 and detector 270. The first MMIC chip 250combines the hybrid LNA (approximately 20 decibel gain and less than 4dB NF) configuration with a non-reflective switch 208 of less than about1.5 dB. The second LNA chip 260 can be a single channel amplifier ofabout 16 DB gain which provides the capability of adjusting gain bychanging the gate bias (Vg). The third MMIC chip 266 combines a singlechannel low noise amplifier with a detector diode 270.

It is evident that the radiometer of the present invention enhancesperformance and reduces complexity. The radiometer includes the benefitsof the hybrid radiometer performance with the simplicity of singlechannel processing provided in the Dicke radiometer. A MMIC chipimplementation is provided that simplifies the RF front endimplementation. One MMIC chip combines the LNA hybrid function with anon-reflective switch and the second LNA is a simple single channelamplifier. The third LNA combines the amplifier function with a zerobias detector diode to provide a low cost, high performance radiometer.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A MMIC comprising: a hybrid and amplifier circuit that is adapted toreceive an unknown signal and known reference signal and comprising afirst quadrature hybrid as an input having at least one radio frequency(RF) input and parallel signal path outputs; at least one amplifierconnected to each signal path output of the first quadrature hybrid; asecond quadrature hybrid connected to the at least one amplifier at eachsignal path and having parallel RF outputs, wherein the amplifiersprovide equalized amplifier gain; a switch positioned after the dualchannel quadrature hybrid network and operatively connected to theparallel RF outputs of the second quadrature hybrid for selecting one ofthe RF outputs and providing a single RF output; and a detectoroperatively connected to the switch for detecting the single RF outputfrom the switch.
 2. A MMIC according to claim 1, wherein said switchcomprises a non-reflective switch.
 3. A MMIC according to claim 1,wherein said switch is operative for minimizing any amplifier noise. 4.A MMIC according to claim 1, wherein said detector comprises a diode. 5.A MMIC according to claim 1, wherein said detector is ground connected.6. A MMIC according to claim 1, and further comprising dual RF inputsand RF outputs.